A. Field of Invention
The present invention relates generally to microphones and more specifically to microphone electronics and circuits.
B. Description of the Related Art
Typical capacitor microphones include a microphone circuit 10 having a capacitor microphone capsule 12, an impedance converter 14, a phase splitter 16, and two output buffer amplifiers 18, 20, as shown in FIG. 1 of the prior art. Two output buffer amplifiers 18, 20 are generally needed because the output 22 of professional microphones is usually differential and impedance balanced. These output signals are subtracted in the microphone preamplifier or mixing console to minimize the effects of cable capacitance as well as to cancel the common-mode noise signals that may be electromagnetically induced into the microphone cable, which connects the microphone to the preamplifier or mixing console. This technique is often used and well known in the art. The microphone capsule 12 is either biased externally, or in the case of an electret capacitor capsule, biased internally by a static electrical charge. Capacitor microphone capsules are well known in the art.
FIG. 2 of the prior art shows a simplified embodiment of the capacitor microphone circuit 10. The impedance converter 14 is usually comprised of a low-noise field-effect transistor (FET) with the gate bias fed via a very large value resistor Rb, usually about 1G ohm. This large value resistor is necessary to preserve as much of the high-impedance signal from the microphone capsule 12 as possible to maximize signal-to-noise ratio. The phase splitter 16 is often another field-effect transistor arranged in a cathodyne configuration, with equal resistors at the drain and the source. The output buffer amplifiers 18, 20 are often PNP type transistors arranged as emitter followers. Power 24 is usually supplied to the microphone circuit 10 by the mixing console or outboard preamp in a simplex fashion, via the same conductors as the differential output signal. This power arrangement is known in the art as “phantom power.” The microphone circuit 10 includes the voltage source VDD and bias circuits 30.
FIG. 3 of the prior art shows the following modifications which can be made to the microphone circuit 10 to improve signal-to-noise ratio. The output of the FET impedance converter 14 can be fed back to the input of the impedance converter 14 via a bootstrap capacitor 26. This slightly less-than-unity positive feedback helps raise the input impedance of the circuit 10 by cancelling out the loading effect of the FET gate capacitance. Gate capacitance creates a voltage divider with the capsule capacitance and acts as a signal attenuator, which negatively affects signal-to-noise performance. The source resistor of the FET impedance converter can be replaced with a current source 28. This current source 28 has a high AC compliance, which reduces the loading of the FET output signal caused by the FET source resistor Rs. Because the source resistor of the current source 28 can be lower than Rs, the resistor thermal noise contribution can also be reduced. The microphone circuit 10 includes the voltage source VDD and bias circuits 30.
There still remains at least two major sources of noise that limit the signal-to-noise ratio of the circuit 10 in FIG. 3 of the prior art, even if the lowest possible noise FETS are used. The first major source is the thermal noise of the source and drain resistors used in the cathodyne circuit 16. This noise can be reduced by decreasing the value of these resistors; however, reducing these resistors causes an increase in power consumption, which increases as the square of the cathodyne current. The second major source is power supply noise. Any noise present on the voltage source VDD will be algebraically added to the desired signal, thus limiting further signal-noise ratio improvements.
Therefore, what is needed is a method and apparatus for reducing the thermal noise, without appreciably increasing the power consumption, and the power supply noise in microphone electronic circuits.